Downstream treatment of wastewater is a crucial process in ensuring the proper discharge of wastewater into the environment. One of the by-products of this process is the production of biogas， which is a mixture of gases mainly methane and carbon dioxide. Biogas is now seen as a crucial alternative energy source due to its renewable nature and ability to be produced from various organic waste streams.
In many countries， biogas generated from wastewater treatment plants (WWTPs) is used to generate electricity or heat. However， there are still many WWTPs that do not exploit the potential of biogas due to various reasons， including lack of investment， lack of technical expertise， and insufficient incentives. Nevertheless， there are several examples of successful biogas production from WWTPs that can provide guidance for future developments.
The use of anaerobic digestion (AD) is the most common method used for biogas production in WWTPs. AD is a biological process that involves the breakdown of organic materials in the absence of oxygen by microorganisms. The resulting biogas can be used to generate heat and electricity， or it can be purified to create biomethane—a renewable natural gas that can be injected into existing natural gas infrastructure and used as a transport fuel.
In Europe， the EU's Renewable Energy Directive (RED) and its recast， the RED II， have been established with the aim of encouraging the development of sustainable bioenergy， including biogas. The RED II sets a binding target of 14% for the share of renewable energy in transport fuel by 2030. This target will require a significant increase in the production of biomethane， including from WWTPs.
In conclusion， the production of biogas from WWTPs is a significant opportunity to improve the sustainability of the wastewater treatment process while also providing a renewable energy source. It requires investment and technical expertise， but with the increasing focus on renewable energy， biogas from WWTPs has the potential to play a significant role in the energy mix of the future.
